1. Technical Field
The present invention relates generally to voice communication networks; and more particularly to a voice quality performance evaluator that objectively evaluates the performance of vocoders used in conjunction with voice communication networks and/or the voice communication networks themselves.
2. Related Art
Communication networks are well known to facilitate voice communications. A prime example of such a communication network is the public switched telephone network (PSTN). The PSTN has evolved greatly since its original creation and continues to service voice communications worldwide. The PSTN typically employs pulse code modulation (PCM) within a circuit switched network to transmit voice communications between users. The PSTN generally sets a level of performance for voice traffic that is used as a benchmark for evaluating the performance of other networks.
Other communication networks that also service voice communications include packet data networks such as the Internet. Packet data networks convert voice signals between the PCM format (as used with the PSTN) and a packet data format or directly from an analog signal to the packet data format. The packet data is then transmitted across the packet data network and reconverted to the PCM format or directly to an analog format at a terminating end. Packet data networks, because they are packet switched and not circuit switched, cannot guarantee bandwidth between two voice users. When sufficient resources exist to service voice communications, the packet data network provides PSTN like voice quality. However, in typical situations, the packet data network cannot service each voice communication with resources that are sufficient to provide PSTN like voice quality. Thus, voice communication users employing the packet data network oftentimes receive inferior performance as compared to the PSTN.
Other examples of communication networks that service voice communications include wireless system networks. Examples of such wireless system networks are cellular systems and satellite systems. These wireless system networks are typically bandwidth limited on the wireless link between the mobile units and base stations (or satellites). Therefore, vocoders are employed to convert voice signals from a standard PCM format (or analog format) to a bandwidth reduced format. The vocoders use compression techniques to code the voice communications at a transmitting end and decode the voice communications at a receiving end. Such coding and decoding often distorts the voice communication, reducing the quality of service provided by the vocoders and the wireless system networks.
Additional problems in the performance of the wireless system networks also reduce the quality of voice communications. A wireless link between the base station (or satellite) and a mobile unit is typically less than one hundred percent reliable. In analog wireless system networks, static is introduced into the voice signal carried over the wireless link. Further, in digital wireless communication networks, frames are often lost over the wireless links. When frames are lost over the wireless links, the vocoders attempt to provide a continuous voice signal to the user. In providing this continuous voice signal, the vocoder fills in missing information with best guess approximations of the voice signal. Such approximations typically provide marginal performance and produce a noticeable degradation in performance.
Other problems affecting the quality of voice communications in wireless system networks (and in systems using a packet data network for part of the transmission link) relate to the operation of the vocoders themselves. Vocoders are designed based upon the fundamentals of human speech patterns. According to their design, the vocoders attempt to perform coding and decoding to maximize voice quality. However, because assumptions regarding speech patterns are invalid under some operating situations, the vocoders provide substandard performance in the operating situations. Examples of such operating situations include teleconferencing, in which multiple voice signals are incoming simultaneously. Assumptions made with respect to a single voice signal are typically invalid when applied to multiple voice signals.
Because of the many shortcomings in voice communications identified above, it is desirable to grade the voice communication performance of vocoders and communication networks. However, such evaluation is difficult to perform due to the great complexity of the voice communications themselves. Thus, a typical technique for grading the performance of vocoders and communication networks has been to use a panel of observers to subjectively grade the performance of the vocoders and communication networks. From the subjective grades obtained, a Mean Opinion Scoring (MOS) number is obtained for each of a plurality of vocoders and/or communication networks under test. Then, the MOS numbers are averaged and compared to one another to arrive at a relative rank for each device under test.
The MOS technique suffers from many shortcomings. Since the MOS testing requires substantial time to accomplish, the observers must be compensated for their time and must be reliable enough to provide unbiased results over a period of time. Further, because differing panels of observers are used to test different vocoders and systems, test results are generally not reproducible and relative ranks of devices often vary over time.
Thus, there is a need in the art for a device that evaluates the performance of vocoders and/or communication networks for voice communications.